The permanent magnet is one of the most important electrical and electronic materials used in varied application areas ranging from household electric appliances to peripheral equipment of large computers. There is an increasing demand for permanent magnets of high performance to meet a recent requirement for making electric appliances smaller and more efficient than before.
Typical of permanent magnets now in use are alnico magnets, hard ferrite magnets, and rare earth-transition metal magnets. Much has been studied on rare earth-cobalt permanent magnets and rare earth-iron permanent magnets, which belong to the category of the rare earth-transition metal magnets, because of their superior magnetic performance. Rare earth-iron permanent magnets are attracting attention on account of their lower price and higher performance than rare earth-cobalt permanent magnets which contain a large amount of expensive cobalt.
Heretofore, there have been rare earth-iron permanent magnets produced by any of the following three processes.
(1) One which is produced by the sintering process based on the powder metallurgy. (See Japanese Patent Laid-open No. 46008/1984.) PA1 (2) One which is produced by binding thin ribbons (about 30 .mu.m thick) with a resin. Thin ribbons are produced by rapidly quenching the molten alloy using an apparatus for making amorphous ribbons. (See Japanese Patent Laid-open No. 211549/1984.) PA1 (3) One which is produced from the thin ribbons (produced as mentioned in (2) above) under mechanical orientation by the two-stage hot pressing method. (See Japanese Patent Laid-open No. 100402/1985.) PA1 (1) The refining crystal grains at the time of casting. PA1 (2) The formation of the uniform structure after working which is attributable to improved workability.
The present inventors previously proposed a magnet produced from a cast ingot which has undergone mechanical orientation by the one-stage hot working. (See Japanese Patent Application No. 144532/1986 and Japanese Patent Laid-open NO. 276803/1987.) (This process is referred to as process (4) hereinafter.)
The above-mentioned process (1) includes the steps of producing an alloy ingot by melting and casting, crushing the ingot into magnet powder about 3 .mu.m in particle size, mixing the magnet powder with a binder (molding additive), press-molding the mixture in a magnetic field, sintering the molding in an argon atmosphere at about 1100.degree. C. for 1 hour, and rapidly cooling the sintered product to room temperature. The sintered product undergoes heat treatment at about 600.degree. C. to increase coercive force.
In the above-mentioned process (2) rapidly cooled thin ribbons of R--Fe--B alloy are produced by a melt-spinning apparatus at an optimum substrate velocity. The rapidly cooled thin ribbon is about 30 .mu.m thick and is an aggregation of crystal grains 1000.ANG. or less in diameter. It is brittle and liable to break. It is magnetically isotropic because the crystal grains are distributed isotropically. To make a magnet, this thin ribbon is crushed into powder of proper particle size, the powder is mixed with a resin, and the mixture undergoes press molding.
According to the above-mentioned process (3), the thin ribbon obtained by the process (2) undergoes mechanical orientation by a two-stage hot pressing in vacuum or an inert gas atmosphere. Thus there is obtained a anisotropic R--Fe--B magnet. In the pressing stage, pressure is applied in one axis so that the axis of easy magnetization is aligned in the direction parallel to the pressing direction. This alignment process brings about anisotropy. This process is executed such that the crystal grains in the thin ribbon has a particle diameter smaller than that of crystal grains which exhibit the maximum coercive force, and then the crystal grains are designed to grow to a optimum particle diameter during hot-pressing.
The above-mentioned process (4) is designed to produce and anisotropic R--Fe--B magnet by hot-working an alloy ingot in vacuum or an inert gas atmosphere. The process causes the axis of easy magnetization to align in the direction parallel to the working direction, resulting in anisotropy, as in the above-mentioned process (3). However, process (4) differs from process (3) in that the hot working is performed in only one stage and the hot working makes the crystal grains smaller.
The above-mentioned prior art technologies enable to produce the rare earth-iron permanent magnets; but they have some drawbacks as mentioned below.
A disadvantage of process (1) stems from the fact that it is essential to finely pulverize the alloy. Unfortunately, the R--Fe--B alloy is so active to oxygen that pulverization causes severe oxidation, with the result that the sintered body unavoidably contains oxygen in high concentrations. Another disadvantage of process (1) is that the powder molding needs a molding additive such as zinc stearate. The molding additive is not able to be removed completely in the sintering step but partly remains in the form of carbon in the sintered body. This residual carbon considerably deteriorates the magnetic performance of the R--Fe--B permanent magnet. An additional disadvantage of process (1) is that the green compacts formed by pressing the powder mixed with a molding additive are very brittle and hard to handle. Therefore, it takes much time to put them side by side regularly in the sintering furnace.
On account of these disadvantages, the production of sintered R--Fe--B magnets needs an expensive equipment and suffers from poor productivity. This leads to a high production cost, which offsets the low material cost.
A disadvantage of processes (2) and (3) is that they need a melt-spinning apparatus which is expensive and poor in productivity. Moreover, process (2) provides a permanent magnet which is isotropic in principle. The isotropic magnet has a low energy product and a hysteresis loop of poor squareness. It is also disadvantageous in temperature characteristics for practical use.
A disadvantage of process (3) is poor efficiency in mass production which results from performing hot-pressing in two stages. Another disadvantage is that hot-pressing at 800.degree. C. or above causes coarse crystal grains, which lead to a permanent magnet of impractical use on account of an extremely low coercive force.
The above-mentioned process (4) is the simplest among the four processes; it needs no pulverization step but only one step of hot working. Nevertheless, it has a disadvantage that it affords a permanent magnet which is a little inferior in magnetic performance to those produced by process (1) or (3).